A fuel cell is a device in which a fuel such as hydrogen or methanol is electrochemically oxidized in the cell and the chemical energy of the fuel is thereby converted directly into electrical energy to be taken out. Recently, fuel cells have attracted attention as clean supply sources of electrical energy. In particular, solid polymer fuel cells that use proton conducting membranes as electrolytes are expected to be promising as electrical power supplies for electric automobiles because such fuel cells are high in output power density and capable of being operated at low temperatures.
The fundamental structure of such a solid polymer fuel cell has a structure which is constituted with an electrolyte membrane and a pair of gas diffusion electrodes each having a catalyst layer and respectively joined to both sides of the electrolyte membrane, and further with two current collectors respectively disposed on the gas diffusion electrodes. Thus, one gas diffusion electrode (anode) is fed with hydrogen or methanol as a fuel, and the other gas diffusion electrode (cathode) is fed with oxygen or air as an oxidant; an external load circuit is connected between both gas diffusion electrodes, and this structure thereby operates as a fuel cell. In this case, the protons generated in the anode migrate through the electrolyte membrane to the cathode side to react with oxygen to generate water. Thus, the electrolyte membrane functions as a migration medium for protons and a diaphragm for hydrogen gas and oxygen gas. Accordingly, a polymer electrolyte membrane for a fuel cell is required to be high in proton conductivity, strength and chemical stability, and further to be excellent in gas barrier property.
In this connection, what are conventionally referred to as functional membranes suffer from problems such that functional groups are distributed randomly over a whole membrane, and in the cases of labyrinth membranes and mesh membranes having functional groups, it is impossible to control the spatial distribution of the functional groups and the functional group density.
Specifically, commercially available electrolyte membranes such as Nafion (trade name) and solid polymer electrolyte membranes produced by means of radiation graft polymerization each have hydrophilic cation exchange groups uniformly distributed in the interior of the membrane, hence are swollen by excessive hydration to weaken the intermolecular interaction, and consequently, suffer from the occurrence of excessive crossover of hydrogen or methanol. Additionally, for example, Gore Co. and Tokuyama Co., Ltd. have made some attempts to fill porous membranes having three-dimensionally continuous pores of extremely high porosity with ion exchange resins; however, some ion exchange resin fractions are not involved in cation exchange to result also in extreme swelling. Further, the porous substrate to be used is limited to polytetrafluoroethylene and polyethylene capable of being made to be porous, and these polymers fail to meet the gas barrier property required for the electrolyte membranes for use in fuel cells, so that the above-mentioned property of the solid polymer electrolyte membrane thus obtained has been found to be insufficient in the light of the properties required for fuel cells.